1. Field of the Invention
The present invention relates generally to a magnetic fluxgate (or flux valve) direction sensor (or flux sensor) for detecting the direction and magnitude of a magnetic field. Fluxgate sensors are used commercially in compass and direction bearing equipment.
The present invention is applicable to detect the direction and magnitude of a weak magnetic field such as geomagnetism.
The present invention relates more specifically to a thin film compass and a method for the fabrication of the same using thin film micro-fabrication technology, wherein improvements have been in the uniformity of the core cross section and excitation coil winding, in the precise alignment of the sensing coil windings, and in the reduction of the physical size of the sensor.
2. Description of the Prior Art
A prior geomagnetism fluxgate sensor has been shown in the M. F. Lynch et al. U.S. Pat. No. 4,262,427 for "Flux Valve Compass System," in the S. Arakawa et al. U.S. Pat. No. 4,616,424 for "Magnetic Direction Sensor," and in the K. Mohri et al. U.S. Pat. No. 4,739,263 for "Magnetic Sensor Using The Earth's Magnetism."
An example of such a fluxgate sensor is shown in FIG. 1. In the figure, the sensor has an O-ring shaped magnetic core C, on which an excitation coil Cc is wound. A pair of sensing coils Cx and Cy are also wound on the magnetic core so that each coil encircles the opposite portion of the magnetic core. The sensing coils are arranged perpendicular to each other so that they provide output voltage signals to determine the angle between the magnetic core and an external magnetic field whenever the excitation coil is energized by a pulse generator. The magnitude of the external magnetic field can also be determined from the output voltage signals.
Other than the O-ring shaped magnetic core, a known flux valve on earth's field directional pickoff is a structure having an equiangular "Y"-shaped core, as disclosed in the M. C. Depp U.S. Pat. No. 2,852,859 for "Flux Valve Compensating System." An example of such a fluxgate sensor is shown in FIG. 2. In C. F. Fragola U.S. Pat. No. 2,389,146 for "Flux Valve," a dual-core system was disclosed. Generally, the ring-shaped core is the most common configuration, which is made of materials having a high magnetic permeability, such as permalloy, ferromagnetic materials, and zero-magnetostrictive amorphous magnetic materials. This kind of core may be formed by winding a thin ribbon, or by bending a single or a bundle of thin wires, or using bars to form a circle or a polygon.
The magnetic fluxgate sensor of the ring-shaped core type is relatively good in detection sensibility. Its structure is simple and its subsequent signal-treating circuit is relatively simple. However, it has the following disadvantages due to the present manufacturing processes:
First, it is difficult to fabricate a magnetic core with uniform cross section to ensure a constant magnetic flux.
Further, automatic winding of a uniform excitation coil on a ring-shaped core is difficult, and non-uniform coil winding will cause a magnetic azimuth error to occur.
Further, it is difficult to obtain a uniform winding distribution in the sensing coils. When this happens, the directions of the input axis of the sensing coils are displaced from the desired direction with the result that a magnetic azimuth error occurs.
Further, the sensor is too large in size for practical use (for example, a prior sensor has a diameter of 10-50 mm, and a thickness of 3 mm).
Further, a permalloy core, if not protected and supported, is easily broken by vibration and/or shock.
A number of solutions have been proposed to alleviate the above problems. In U.S. Pat. No. 4,763,072 to Katoh et al., for example, a magnetic azimuth detector is proposed including an annular iron core and an annular hollow bobbin for a primary winding which incorporates therein the annular iron core. The bobbin has a number of protruded portions distributed evenly along a circumference of the annular bobbin to divide the circumference of the bobbin into sections of equal length between the protruded portions. The primary windings are wound around these sections of equal length with equal number of turns and in the same direction. Two secondary winding bobbins are shaped to fit on the two sides of the bobbin for the primary winding. Each of the two secondary winding bobbins has positioning means (e.g., partitioning protrusions) for registering the positions of the secondary winding bobbins with respect to the primary winding bobbin. The partitioning protrusions are also evenly distributed along the circumferences of the secondary winding bobbins to divide the circumferences into compartments of equal length, and the secondary windings are wound around compartments of equal length with an equal number of turns. It is stated in Katoh et al. that, by dividing the primary winding into smaller segments, it is easier to uniformly wind the winding around such small portions. By providing positioning means between the primary winding bobbins and the secondary winding bobbins, it is stated in Katoh et al. that it is possible to reduce the error of the magnetic sensor. Katoh et al.'s design is disadvantageous since large mechanical devices such as bobbins are required and small compasses cannot be made using such design. Furthermore, because of the difficulty in winding wires around bobbins accurately along predetermined paths, it would still be difficult to significantly reduce magnetic azimuth errors using Katoh et al.'s technique.
In U.S. Pat. No. 4,739,263 to Mohri et al. referenced above, another direction sensor is proposed comprising a ring-shaped core formed from at least one flexible wire and two pairs of series connected coils spaced from each other at equal intervals about the core for sensing an external magnetic field. The core is formed by threading the wires through the coils. Eventhough the wire used by Mohri et al. has smaller dimensions than the bobbins of Katoh et al., Mohri et al.'s design is still too big or too thick for many applications. Moreover, it is more difficult to wind wires accurately around another wire as compared to winding wires around larger mechanical supports such as bobbins, and to accurately fabricate sensor cores in the shape of circles or polygons by bending wires.
None of the above-described magnetic direction sensors or compasses is entirely satisfactory. It is therefore desirable to provide an improved magnetic field direction sensor or compass in which the above-described difficulties are overcome.